Originally Posted by GBR in WA
Adding insulation adds weight to the flex ducting. At the initial install, the supporting strap spacing (eg.- 5') may have been suitable. You may need closer strapping afterward because if it sags more = a reduction of air-flow because of kinking at the straps;http://www.thefreelibrary.com/Pressu.....-a0201591046
Duct insulation rather than wall cavity insulation is made for them.
I'll move you to HVAC for more responses.
Good point, good info, and thanks for the move.
As an experiment, I wrapped about 85% of one 22' R4.2 duct with some R4.2 and R6 insulation left over from other flex ducts. I'm measuring about 1.5-2 F improvement when the attic is 120+ F, which I expect might improve a little bit with properly installed R6 wrap. That's about a 15% improvement in my split. I'm going to measure it for a few more days then take it all off and measure again before deciding if it's worth it to pursue this. I have another duct of similar length a few feet over that I replaced with a new R6 duct a few years ago I'm also comparing to.
Sweating hasn't been a problem (for the duct, not me) so I guess I could install it a few pieces at a time over a few months if necessary. I've got even longer runs of 40-50' of this horrible flex that would benefit even more. The master bedroom is a degree or two warmer than the rest of the house due to the inadequately insulated flex.
For a number of reasons (access, proximity to other ducts, etc), I may not be able to wrap continuously from start to finish on a given duct. I don't want to do anything that might freak an inspector out, but surely this wouldn't be a problem if the ends are sealed, even though it might look kind of funny in places, right? Considering what did pass inspection, I'm probably worrying a lot more than necessary.